Definition
Inclusio (Latin) is a term of Roman and civil law origin meaning inclusion, enclosure, or the act of shutting in. In legal usage, it appears almost exclusively as part of the maxim inclusio unius est exclusio alterius — the inclusion of one thing is the exclusion of the other — a canon of construction holding that when a legal instrument expressly names certain persons, things, or categories, those not named are presumed to be excluded.
The term itself carries three distinct senses in the historical civil law sources:
1. The physical act of enclosing or including something — as in the setting of a gem within a mount, or the enclosure of bees in a hive.
2. Confinement of a person — the shutting up of an individual in a prison or place of custody.
3. As a logical-legal concept — the starting premise for the inclusio/exclusio maxim that operates across statutory interpretation, contract construction, and common law pleading.
In modern legal writing, inclusio almost never appears as a standalone term. Its significance is entirely derived from the maxim it anchors.
Common Confusion
INCLUSIO UNIUS and EXPRESSIO UNIUS: These are two names for the same maxim. Anderson's Dictionary of Law cross-references INCLUSIO directly to EXPRESSIO, reflecting the standard practice of treating them as interchangeable. In modern usage, expressio unius est exclusio alterius is the dominant form encountered in case law and secondary sources. Researchers searching historical sources may find the inclusio unius formulation used with equal frequency in civil law-influenced jurisdictions and treatises. The underlying interpretive principle is identical: express mention of one implies exclusion of others. Neither formulation is more authoritative than the other; the variation is stylistic and jurisdictional.
Why It Matters in Research
Researchers encountering inclusio in historical sources should treat it as a pointer to the exclusio alterius interpretive canon, not as a substantive doctrinal term with independent content. The word's standalone appearances in the corpus are almost entirely in two contexts: (1) civil law treatises discussing Roman property concepts — gem-setting, beekeeping, and imprisonment — where the physical sense of enclosure governs; and (2) maxim collections and interpretive canons sections, where it functions as the first half of the inclusio unius est exclusio alterius formula.
The civil law sources cited by Burrill — Heineccius's Elementa Juris Civilis and Fleta — use the term in its literal, physical sense. Researchers working on Roman law property questions (accession, usucaption, enclosure of animals as a method of acquisition) will find these passages relevant. Researchers working on statutory construction or contract interpretation should pivot immediately to the exclusio alterius maxim literature and to expressio unius entries.
One trap worth flagging: in older treatises and digests, the maxim is sometimes cited in truncated form as simply inclusio unius, leaving the exclusio alterius half implied. This can create indexing gaps when searching historical corpus materials by maxim text. Search both the inclusio unius and expressio unius forms to capture the full range of authority.
The canon itself has had a contested reception in American courts. Some jurisdictions treat it as a strong presumption; others treat it as a weak default easily overcome by context. Researchers tracing the canon's authority across time will find the historical dictionary sources largely uncritical of it — the modern scholarly debate about its reliability as an interpretive tool is not reflected in Anderson or Burrill.
Historical Dictionary Support
Burrill provides the most substantive entry, tracing the Latin root (includere — to shut in, enclose, include) and supplying three distinct senses drawn from Heineccius and Fleta. The Fleta citations are precise: the beekeeping sense appears at lib. 3, c. 2, § 3, and the imprisonment sense at lib. 2, c. 65, § 7. These are genuine medieval English legal compilations drawing on Roman civil law, and the citations are traceable. Burrill also begins the text of the maxim — "Inclusio unius est exclusio alterius. The inclusion..." — though the entry in the available source appears truncated before the maxim's full exposition.
Anderson, by contrast, dispenses with the term almost entirely, directing the reader to EXPRESSIO. This is editorially telling: by the time of Anderson's compilation, expressio unius had become the dominant form in common law usage, and inclusio unius was receding to the status of a civil law variant. The two dictionaries together illustrate the transitional moment when civil law vocabulary was being absorbed into common law interpretive doctrine under anglicized terminology.
Neither source engages with the interpretive limits of the maxim — both treat it as settled and reliable. Modern treatises on statutory construction, including those by Sutherland and later commentators, offer the critical perspective that these historical sources lack.
Jurisdictional Note
Civil law jurisdictions and states with strong civilian traditions (Louisiana, to a degree) are more likely to encounter the inclusio unius formulation in older authorities. Common law jurisdictions overwhelmingly use expressio unius in modern practice. The underlying canon operates across both traditions, though its weight varies by jurisdiction and context.